Inverter terminal voltage adjustment in power system

ABSTRACT

A utility-scale energy storage and conversion system can operate two or more inverter groups such that their reactive power commands are proportional to their available reactive power range. The control system can therefore distribute the reactive power commands in proportion to the available Q range, thereby ensuring that all inverters in the utility-scale energy storage and conversion system 100 operate with the same Q “headroom”. In addition, the utility-scale energy storage and conversion system can use an on-load tap changer (LTC) to adjust a terminal voltage associated with a first group of inverters and a second group of inverters. The first group of inverters can be associated with a first rating and the second group of inverters can be associated with a second rating that is greater than the first rating.

PRIORITY APPLICATIONS

This application is a divisional of U.S. Pat. Application Serial No.17/302,615, filed May 7, 2021, the content of which is incorporatedherein by reference in its entirety.

FIELD OF THE DISCLOSURE

This document relates generally to electric power generation systems andmore particularly, but not limited to, regulation of power in electricpower generation systems.

BACKGROUND

The key component determining the rating of a building block is thepower electronic inverter used to interface energy storage containers(using DC), e.g., batteries, to an electric power system (using AC).Because of their functional complexity and the associated lengthycertification processes, the ratings of the inverter are inflexible. Asa result, once an inverter is selected, getting cost-efficiency for theproject is equivalent to maximizing the utilization of the selectedinverters.

Conventionally, utility-scale energy storage projects are built usingwhole-number multiples of identical building blocks. Manufacturingeconomies of scale result in per-unit costs of building-blocks beinginversely proportional to their size, making the projects with a fewlarge blocks more cost-effective than those with many small blocks.However, these advantages can be quickly reset if a design must beoversized on an account of the first whole-number building-block ratingmultiple being significantly larger that the desired project rating. Forexample, if a target is to generate a rating of 9 using multiples of ablock sized at 4, one must choose 3 blocks, resulting in the totalrating being 12 instead of 9 and an effective cost penalty of 33.3%.

SUMMARY OF THE DISCLOSURE

This disclosure describes a utility-scale energy storage and conversionsystem that can operate two or more inverter groups such that theirreactive power commands are proportional to their available reactivepower range. The control system can therefore distribute the reactivepower commands in proportion to the available Q range, thereby ensuringthat all inverters in the utility-scale energy storage and conversionsystem 100 operate with the same Q “headroom”. In addition, theutility-scale energy storage and conversion system can use an on-loadtap changer (LTC) to adjust a terminal voltage associated with a firstgroup of inverters and a second group of inverters. The first group ofinverters can be associated with a first rating and the second group ofinverters can be associated with a second rating that is greater thanthe first rating.

In some aspects, this disclosure is directed to a utility-scale energystorage and conversion system coupled to an electric network at a pointof interconnect, the system comprising: a first group of batterieshaving a first storage capacity; a second group of batteries having asecond power storage capacity different from the first storage capacity;a first group of inverters in electric communication with the firstgroup of batteries, the first group of inverters having a first rating;and a second group of inverters in electric communication with thesecond group of batteries, the second group of inverters having a secondrating, wherein the second rating is the same as the first rating,wherein outputs of the first group of inverters and the second group ofinverters are combined and supplied to the electric network at the pointof interconnect.

In some aspects, this disclosure is directed to a utility-scale energystorage and conversion system that adjusts a terminal voltage associatedwith a first group of inverters and a second group of inverters, whereinthe first group of inverters is associated with a first rating, whereinthe second group of inverters is associated with a second rating that isgreater than the first rating, the utility-scale energy storage andconversion system comprising: a control circuit to: determine, using arepresentation of a first available reactive power value associated withthe first group of inverters and a representation of a second availablereactive power value associated with the second group of inverters, arepresentation of a total available reactive power; generate, using therepresentation of the first available reactive power value and arepresentation of a total required reactive power value, a first controlsignal representing a first desired reactive power value proportional tothe first available reactive power value; and generate, using therepresentation of the second available reactive power value and thetotal required reactive power value, a second control signalrepresenting a second desired reactive power value proportional to thesecond available reactive power value.

In some aspects, this disclosure is directed to a utility-scale energystorage and conversion system to use an on-load tap changer (LTC) toadjust a terminal voltage associated with a first group of inverters anda second group of inverters, wherein the first group of inverters isassociated with a first rating, wherein the second group of inverters isassociated with a second rating that is greater than the first rating,the utility-scale energy storage and conversion system comprising: acontrol circuit to: determine, using a representation of a measuredcurrent and a representation of a measured voltage at a transformerwinding of a transformer in the system, the terminal voltage associatedwith the first group of inverters and the second group of inverters;determine, using a representation of a total required reactive powervalue, a target terminal voltage associated with the first group ofinverters and the second group of inverters; and generate, using adifference between the terminal voltage and the target terminal voltage,a tap setting on the LTC so that the terminal voltage associated withthe first group of inverters and the second group of inverters adjustsaccordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 is a one-line diagram of an example of a utility-scale energystorage and conversion system that can implement various techniques ofthis disclosure.

FIG. 2 is a conceptual diagram of an example of a control circuit thatcan implement various techniques of this disclosure.

FIG. 3 is a conceptual diagram of an example of a control circuit thatcan implement various techniques of this disclosure.

FIG. 4 is a graph of an example of a transfer function that relates atotal required reactive power value and a target terminal voltage.

DETAILED DESCRIPTION

The primary function of utility-scale energy storage installations is toexchange active power with the power system for purposes of maintainingsystem frequency or arbitraging energy. In addition to this primaryfunction, utility-scale energy storage installations are also requiredto maintain electrical conditions at the point of interconnect (POI)within an operating range. Most commonly this requires maintaining thePOI voltage, which in turn requires supplying reactive power to thesystem. Reactive power can be supplied from dedicated sources, such asswitched capacitors, or from the inverters already utilized to providethe primary function of interfacing energy storage elements (outputtingDC voltage), to an AC system.

The present inventors have recognized an opportunity to minimizeinstallation complexity and, consequently, cost by meeting both theprimary function of utility-scale energy storage installations and theneed to maintain electrical conditions at the POI from the invertersused to interface energy storage elements. The cost is further reducedby controlling the system in a way that considers available reactiverange of inverters given their primary function of power delivery and bykeeping the inverters at their maximum effective ratings by controllingtheir terminal voltage using an on-load tap changer (LTC).

As described in more detail below, various techniques of this disclosurecan also leverage an on-load tap changer (LTC) to control dynamicreactive power from the battery inverters to meet the voltagerequirements at the POI. For each LTC operation, the inverter’s realpower (P) and reactive power (Q) outputs are readjusted to try tomaintain the POI voltage at a value of interest. For calculations of thecorner points of capability, the final tap position of the LTC can beheld, and the Q outputs of the inverters can be changed until bothinverters reach the current limit. Using the LTCs to control theinverters’ terminal voltages can help ensure that the grid voltage doesnot affect the dynamic reactive range of the plant. Also, using a“directionally sensitive setpoint” can help to ensure that the batteryinverters can stay within the allowed terminal voltage range duringsudden changes in reactive output.

FIG. 1 is a one-line diagram of an example of a utility-scale energystorage and conversion system that can implement various techniques ofthis disclosure. The utility-scale energy storage and conversion system100 can include a first group of batteries 102A and a second group ofbatteries 102B. The first group of batteries 102A can have a firststorage capacity and the second group of batteries having a second powerstorage capacity different from the first storage capacity, such asgreater than the first power storage capacity.

The system 100 can include a first group of inverters 104A in electriccommunication with the first group of batteries 102A and a second groupof inverters 104B in electric communication with the second group ofbatteries 102B. In some examples, the first group of inverters 104A caninclude a first number of inverters, and the second group of inverterscan include a second number of inverters different than the firstnumber. The battery and inverter combinations can help regulate thefrequency at the point of interconnect.

Because the first group of inverters 104A is associated with the firstgroup of batteries 102A, the first group of inverters 104A can beassociated with a first rating. Similarly, because the second group ofinverters 104B is associated with the second group of batteries 102B,the second group of inverters 104A can be associated with a secondrating that is different than the first rating, such as greater than thefirst rating.

In some examples, the inverters in each of the two groups 104A, 104B canhave the same rating. In one non-limiting example, individual ones ofthe inverters in the two groups 104A, 104B can have identical ratings.In another non-limiting example, individual ones of the inverters in thetwo groups 104A, 104B can have different ratings, but the groups 104A,104B can have the same rating. Likewise, in some examples, each group104A, 104B can have the same number of inverters within each group 104A,104B. However, because the first group of inverters 104A is associatedwith a first group of batteries 102A that has a different energy storagecapacity than the second group of batteries 102B associated with thesecond group of inverters 104B, the inverter/battery pairings can beconsidered to be non-homogeneous.

The second group of inverters 104B can output more power because theyare associated with the second group of batteries 102B, which can have ahigher energy storage capacity then the first group of batteries 102A.However, the second group of inverters 104B can have less reactive powerrange than the first group of inverters 104A.

The first group of inverters 104A can be coupled with a first voltagebus LV1, e.g., a low-voltage bus at 400-500 volts (V), and the secondgroup of inverters 104B can be coupled with a second voltage bus LV2,e.g., a low-voltage bus at 400-500V. The voltage of the first voltagebus LV1 can be increased using groups of step-up transformers 106A,e.g., pad-mount transformers, that are coupled with a medium-voltage busMV, e.g., 34.5 kilovolts (kV). Similarly, the voltage of the secondvoltage bus LV2 can be increased using groups of step-up transformers106B, e.g., pad-mount transformers, that are coupled with themedium-voltage bus MV. The impedance of MV cables coupling the groups ofstep-up transformers 106A to the MV bus is represented by impedance Zcs.The group of step-up transformers 106B will be similarly coupled to themedium-voltage bus MV via cabling having an associated impedance Z_(CS).

The voltage of the medium-voltage bus MV can be increased using anotherstep-up transformer, namely main power transformer (MPT) 108. The MPT108 can increase the voltage to a transmission voltage, such as 138kV,at the high-voltage bus HV. The MPT 108 can include an on-load tapchanger (LTC) 109 coupled with taps on a high-voltage winding of the MPT108. The LTC can adjust the number of turns on the high-voltage winding,which can change the turns ratio of MPT 108 and thus adjust the outputvoltage of the transformer. In some examples, the turns ratio of the MPT108 can be adjustable in a +/- 10% range in 16 steps in each direction.One or more meters 110, such as current and/or voltage meters, can becoupled to the low side of the MPT 108 to monitor conditions.

The utility-scale energy storage and conversion system 100 can becoupled with an electric network 112, which is represented by a Theveninequivalent voltage V_(TH) at an infinite bus INF. The utility-scaleenergy storage and conversion system 100 can form part of a firstelectric network and the electric network 112 can be considered a secondelectric network.

The coupling of the electric network 112 with the utility-scale energystorage and conversion system 100 at a point of interconnect (POI) isrepresented by a Thevenin equivalent impedance Z_(TH). The electricalconnection between the high-voltage bus HV of the utility-scale energystorage and conversion system 100 and the POI is represented by animpedance Z_(TIE). In FIG. 1 , the outputs of the first group ofinverters 104A and the outputs of the second group of inverters 104B arecombined at the MV bus and supplied to the electric network 112 at thePOI.

This disclosure describes, among other things, two aspects of theutility-scale energy storage and conversion system 100: 1) anon-homogeneous design of the system 100, and 2) an adaptive voltagemanagement technique to increase effective inverters’ ratings.

The non-homogeneous design of the system 100 divides the inverters intomultiple groups with differently rated energy storage capacity withineach group. As mentioned above, the two inverter groups 104A, 104B areassociated with groups of batteries 102A, 102B, respectively. Assumethat the first group of batteries 102A has the greater energy storagecapacity than the second group of batteries 102B. To charge (ordischarge) the two groups at the same relative pace, the first group ofinverters 104A must run at a higher active power than the second groupof inverters 104B. This uneven active power dispatch leaves the secondinverter group 104B with the greater available reactive range than thefirst group of inverters 104A. So, to fully utilize its inverters’ratings, the second group of inverters 104B is given the higher reactivepower command than the first group of inverters 104A.

Using various techniques of this disclosure, the utility-scale energystorage and conversion system 100 can operate both inverter groups 104A,104B such that their reactive power commands are proportional to theiravailable reactive power range. By way of a non-limiting example, aninverter with the apparent power (S) ratings of 2,500kVA that is given a2,250kW active power (P) command, has the available reactive (Q) rangeof ~1,090kVAr (kVA rating is an orthogonal sum of active and reactivepower). The same inverter operating at a 2,125kW P command (a ~5.5%reduction in P) has the available reactive range of ~1,317kVA (a ~21%increase in Q). The control system can therefore distribute the reactivepower commands in proportion to the available Q range, thereby ensuringthat all inverters in the utility-scale energy storage and conversionsystem 100 operate with the same Q “headroom”.

FIG. 2 is a conceptual diagram of an example of a control circuit thatcan implement various techniques of this disclosure. In some examples,the control circuit 200 of FIG. 2 can form part of or be incommunication with a plant controller for the utility-scale energystorage and conversion system 100 of FIG. 1 . The control circuit 200can operate non-homogeneous inverter groups, such as the inverter groups104A, 104B of FIG. 1 , such that their reactive power commands areproportional to their available reactive power range. The controlcircuit 200 can include, form part of, or be coupled with a processorthat can execute instructions to perform the various functions describedin this disclosure.

The control circuit 200 can receive a signal StotA representing anavailable apparent power value StotA associated with the first group ofinverters 104A of FIG. 1 and a signal StotB representing an availableapparent power associated with the second group of inverters 104B ofFIG. 1 . The control circuit 200 can receive a signal PcmdA representinga required active power value associated with the first group ofinverters 104A of FIG. 1 and a signal PcmdB representing a requiredactive power command associated with the second group of inverters 104Bof FIG. 1 .

Using the relationship [Apparent power (S)]² = [Real Power (P)]² +[Reactive Power (Q)]², the control circuit 200 can determine arepresentation of a total available reactive power Qtot. Moreparticularly, at block 202A, the control circuit 200 can perform themathematical operation of squaring the total available apparent powerStotA associated with the first group of inverters 104A of FIG. 1 . Atblock 204A, the control circuit 200 can perform the mathematicaloperation of squaring the active power command PcmdA associated with thefirst group of inverters 104A of FIG. 1 . At block 206A, the controlcircuit 200 can subtract the square from block 204A from the block 202A.At block 208A, the control circuit 200 can perform the mathematicaloperation of determining the square root of the sum and determine QtotA,which is a representation of the available reactive power associatedwith the first group of inverters 104A of FIG. 1 .

Similarly, at block 202B, the control circuit can perform themathematical operation of squaring the total available apparent powerStotB associated with the second group of inverters 104B of FIG. 1 . Atblock 204B, the control circuit can square the active power commandPcmdB associated with the second group of inverters 104B of FIG. 1 . Atblock 206B, the control circuit 200 can subtract the square from block204B from the block 202B. At block 208B, the control circuit 200 canperform the mathematical operation of determining the square root of thesum and determine QtotB, which is a representation of the availablereactive power associated with the second group of inverters 104B ofFIG. 1 . At block 210, the control circuit 200 can determine, such as bysumming QtotA and QtotB, a representation of a total available reactivepower Qtot for the two groups of inverters 104A, 104B of FIG. 1 .

Then, the control circuit 200 can use QtotA and QtotB to prorate thetotal required reactive power value, which is represented by the signalQcmdMVAr, into desired reactive power values, represented by controlsignals QcmdA and QcmdB. For example, at block 212, the control circuit200 can perform the mathematical operation of division of the valuerepresented by the signal QcmdMVAr, which represents the total requiredreactive power value demanded by the system operator as being requiredat the POI, by the total available reactive power Qtot. That is, thetotal required reactive power value QcmdMVAr can represent an amount ofdesired reactive power at the point of interconnect between theutility-scale energy storage and conversion system 100 and anotherelectric network 112.

At block 214A, the control circuit 200 can perform the mathematicaloperation of multiplication on the result of block 212 with QtotA, whichis the representation of the available reactive power associated withthe first group of inverters 104A of FIG. 1 , to generate a controlsignal QcmdA that represents a desired reactive power value proportionalto the available reactive power value QtotA.

Similarly, at block 214B, the control circuit 200 can perform themathematical operation of multiplication on the result of block 212 withQtotB, which is the representation of the available reactive powerassociated with the second group of inverters 104B of FIG. 1 , togenerate a control signal QcmdB that represents a desired reactive powervalue proportional to the available reactive power value QtotB.

The control circuit 200 can output the two control signals QcmdA, QcmdBto the inverter groups 104A, 104B. Using the two control signals QcmdA,QcmdB, the utility-scale energy storage and conversion system 100 canadjust a terminal voltage associated with the first group of inverters104A and the second group of inverters 104B. Thus, using varioustechniques of this disclosure, the total required reactive power valuedemanded by the system operator as being required at the POI(represented by the signal QcmdMVAr) can be assigned to the two groupsof inverters 104A, 104B in proportion to the available reactive powervalue Qtot to give each group of inverters the same reactive power Q“headroom”. In addition, these techniques can maximize inverterutilization by providing reactive power to the POI by fully utilizingthe available apparent values StotA, StotB of the inverter groups 104A,104B.

It should be noted that although FIG. 1 depicts two groups of invertersand FIG. 2 is described with respect to the two groups of inverters ofFIG. 1 , the disclosure is not limited to two groups of inverters.Rather, the total required reactive power value demanded by the systemoperator as being required at the POI can be assigned to more than twogroups of inverters in proportion to the available reactive power value.

By using the non-homogeneous plant design techniques described abovewith respect to FIG. 2 , for example, the utility-scale energy storageand conversion system 100 can operate both inverter groups 104A, 104Bsuch that their reactive power Q commands are proportional to theiravailable reactive power range. As such, the control circuit 200 candispatch active power commands to the inverter groups 104A, 104B so asto match their different associated energy storage capabilities, namelythose associated with the groups of batteries 102A, 102B. Thesetechniques allow the groups of batteries 102A, 102B to becharged/discharged relative to their energy storage ratings and notrelative to what the capabilities of the inverter groups 104A, 104B,which can allow the groups of batteries 102A, 102B to discharge at thesame rate, for example. Charging and/or discharging batteries ofdifferent energy storage capacity at corresponding and proportionalrates provides enhanced battery balancing and optimized total durationof active and reactive power control.

As mentioned above, in addition to the techniques described above inwhich the total required reactive power value demanded by the systemoperator as being required at the POI can be assigned to two (or more)groups of inverters in proportion to the available reactive power value,this disclosure describes adaptive voltage management techniques toincrease inverters’ ratings. The adaptive voltage management techniquesdescribed in this disclosure can increase the “effective” inverterratings by adjusting the turns ratio on a main power transformer, e.g.,MPT 108 of FIG. 1 , such as to increase the voltage at the inverters’terminals, such as the terminals of the inverters in the groups ofinverters 104A, 104B of FIG. 1 .

An increase in ratings can be achieved by increasing the voltage, whichresults in a proportional increase in the inverter’s apparent powercapability. Continuing with the example from above, increasing theterminal voltage of an inverter rated 2,500kVA to 1.03 per unit (pu),can result in an effective apparent power capability of 2,575kVA. Thus,an inverter that operates at 2,250kW of active power can now reach1,252kVAr, which is a 14.8% increase relative to the Q range it had at a1.0 pu voltage. In other words, an increase in the terminal voltage ofan inverter can result in a free increase in reactive power Q.

Conventionally, a turns ratio on the main power transformer (MPT), suchas the MPT 108 of FIG. 1 , is controlled to maintain a constant voltagelevel at the medium-voltage (MV) bus. The transmission system voltage,e.g., at the HV bus, can change with the prevailing system conditions,and having a degree of freedom to regulate the voltage profile withinthe system 100 can help ensure that the plant’s capabilities areindependent of the conditions of the electric network 112. This can beimportant for meeting grid-code requirements that, among many things,stipulate an operating range for the system 100 reactive power output asa function of voltage at the POI and for the permitted range of powerexport and import (discharging and charging energy storage,respectively).

A shortcoming of the conventional design is that it can allow theinverters’ terminal voltage to be dependent on the voltage drops acrossthe pad-mount transformers, which can vary with operating conditions.Controlling the inverters’ terminal voltage can explicitly ensure thatthe inverters have an apparent power (kVA) capability independent ofeither the power system voltage or the energy storage plant’s, e.g.,system 100, operating conditions and that the maximum kVA capability isavailable in all operating conditions.

In accordance with various techniques of this disclosure, autility-scale energy storage and conversion system, such as theutility-scale energy storage and conversion system 100 of FIG. 1 , canuse an on-load tap changer (LTC), such as the LTC 109 of FIG. 1 , toadjust a terminal voltage associated with a first group of inverters,e.g., the first group of inverters 104A in FIG. 1 , and a second groupof inverters, e.g., the second group of inverters 104B in FIG. 1 . Thefirst group of inverters can be associated with a first rating and thesecond group of inverters can be associated with a second rating that isgreater than the first rating.

FIG. 3 is a conceptual diagram of an example of a control circuit thatcan implement various techniques of this disclosure. In some examples,the control circuit 300 of FIG. 3 can form part of or be incommunication with a plant controller for the utility-scale energystorage and conversion system 100 of FIG. 1 . The control circuit 300can operate an LTC, such as the LTC 109 of FIG. 1 , to adjust a terminalvoltage associated with a first group of inverters, e.g., the firstgroup of inverters 104A in FIG. 1 , and a second group of inverters,e.g., the second group of inverters 104B in FIG. 1 . In some examples,the control circuit 300 of FIG. 3 and the control circuit 200 of FIG. 2can be the same control circuit or be in communication with one another.The control circuit 300 can include, form part of, or be coupled with aprocessor that can execute instructions to perform the various functionsdescribed in this disclosure.

The control circuit 300 can receive several signals, including PMV, VMV,and QMV. The signal PMV can represent the measured active power to themedium voltage winding of the MPT 108 of FIG. 1 , such as on a per unitbasis. The signal QMV can represent the measured reactive power to themedium voltage winding of the MPT 108 of FIG. 1 , such as on a per unitbasis. The signal VMV can represent the measured voltage magnitude at atransformer winding of a transformer, such as the winding of the MPT 108in FIG. 1 coupled with the MV bus, such as on a per unit basis.

At block 302, the control circuit 300 can perform the mathematicaloperation of division of PMV by VMV to determine a representation of ameasured real current Id. Similarly, at block 304, the control circuit300 can perform the mathematical operation of division of QMV by VMV todetermine a representation of a measured reactive current Iq.

Next, the control circuit 300 can determine a terminal voltage VINVassociated with a first group of inverters and a second group ofinverters, such as the inverter groups 104A, 104B in FIG. 1 , using arepresentation of a measured current and a representation of a measuredvoltage at a transformer winding of a transformer. In some examples, therepresentation of the measured current and the representation of themeasured voltage are measured at a low side of the main powertransformer MPT 108 of FIG. 1 .

At block 306, the control circuit 300 can perform the mathematicaloperation of multiplication of the determined real current Id and theresistance of the step-up transformer 106A of FIG. 1 . At block 308, thecontrol circuit 300 can perform the mathematical operation ofmultiplication of the determined reactive current Iq and the reactanceof the step-up transformer 106A of FIG. 1 . At block 310, the controlcircuit 300 can sum the results of the two multiplications with thesignal VMV and output the result to block 312.

At block 314, the control circuit 300 can perform the mathematicaloperation of multiplication of the determined real current Id and thereactance of the step-up transformer 106A of FIG. 1 . At block 316, thecontrol circuit 300 can perform the mathematical operation ofmultiplication of the determined reactive current Iq and the resistanceof the step-up transformer 106A of FIG. 1 . At block 314, the controlcircuit 300 can sum the results of the multiplications and output theresult to block 312. At block 312, the control circuit 300 can determinethe terminal voltage VINV, where the terminal voltage VINV is given byEquation 1 below:

$\begin{matrix}{\text{VINV} = \text{VMV} + \left( \text{R+jX} \right)\text{*IMV}} & \text{­­­Equation 1}\end{matrix}$

where IMV = conj(Pcmd + jQcmd)/VMV, where VMV has an assumed phasorangle of zero, where the value of Pcmd represents a required activepower value, and where the value of Qcmd represents the total requiredreactive power value (similar to QcmdMVAr).

Next, at block 318, the control circuit 300 can determine, using therepresentation of the total required reactive power value Qcmd, a targetterminal voltage Vcenter associated with the first group of invertersand the second group of inverters, such as the first group of inverters104A and the second group of inverters 104B in FIG. 1 . The targetterminal voltage Vcenter is the desired voltage at the terminals of thefirst group of inverters 104A and the second group of inverters 104B inFIG. 1 .

In some examples, at block 318, the control circuit 300 can implement atransfer function, such as stored in a memory device 320, that relatesthe total required reactive power value Qcmd and the target terminalvoltage Vcenter to determine the target terminal voltage Vcenter. Thememory device 320 can form part of or be in communication with thecontrol circuit 300. At least a portion of the transfer function caninclude a linear relationship between the total required reactive powervalue Qcmd and the target terminal voltage Vcenter, such as shown inFIG. 4 . In some examples, the control circuit 300 can determine thetransfer function using a data set 322 stored in the memory device 320.In some examples, the data set 322 can be stored as a lookup table.

Next, in some non-limiting examples, the control circuit 300 cangenerate, using a difference between the terminal voltage VINV and thetarget terminal voltage Vcenter, a tap setting Tap on the LTC to adjustthe terminal voltage associated with the first group of inverters andthe second group of inverters accordingly. The values of Vcenter andVINV can be delivered to an LTC controller 340 to adjust the tapposition on the MPT. In some examples, the LTC controller can be acommercial off-the-shelf unit working in a manner described in referenceto blocks 320 to 330. At block 324, the control circuit 300 can performa mathematical operation, such as subtraction, to determine a differenceVerr between the terminal voltage VINV and the target terminal voltageVcenter. At block 326, the control circuit 300 can perform athresholding operation to determine whether to adjust the tap setting.For example, if Verr is greater than a first threshold value, thecontrol circuit 300 can adjust the tap setting upward, if Verr is lessthan a second threshold value, the control circuit 300 can adjust thetap setting downward, and if Verr is between the first and secondthresholds, then the control circuit 300 does not adjust the tapsetting.

In some examples, at block 328, the control circuit 300 can perform themathematical operation of integration. Integration can be used toimplement LTC up/down counting. The integrator output of block 328 canbe converted to a nearest integer value at block 330 to model a discretetap position in the LTC, resulting in a tap setting Tap on the LTC, suchas the LTC 109 of FIG. 1 .

In this manner, the control circuit 300 can use the LTC to control thevoltage at the output terminals of the inverters, which can control thevoltage at the POI. Using these techniques, the control circuit 300 canadjust the voltage at the MV bus in FIG. 1 to help ensure that thedesired real and reactive power is available at the POI. Thesetechniques are invariant to grid variations at the POI.

It should be noted that by using these techniques, the LTC, such as theLTC 109 of FIG. 1 , can determine what the voltage magnitude should beon the LV bus and can control those voltages indirectly, rather thanonly considering the voltage magnitude on the MV bus to which the LTC iscoupled. Then, the LTC can control a regulated voltage profile, such asthe real power P and the reactive power Q, at the LV bus. As such, theadaptive voltage management techniques of this disclosure allow the LTCto control a remote bus, such as LV1 and LV2 in FIG. 1 .

FIG. 4 is a graph of an example of a transfer function that relates atotal required reactive power value Qcmd and a target terminal voltageVcenter. The x-axis of the graph 400 represents the total requiredreactive power value Qcmd on a per unit basis, and the y-axis representsthe target terminal voltage Vcenter on a per unit basis. In someexamples, the transfer function 402 can include at least one linearregion 404, such as between points 406, 408. The transfer function 402is not limited to linear relationships and can include a second orhigher order relationship between the total required reactive powervalue Qcmd and the target terminal voltage Vcenter. In other examples,the transfer function 402 can utilize the required active power valuePcmd input in addition to the total required reactive power value Qcmdto generate the target terminal voltage Vcenter.

Various Notes

Each of the non-limiting aspects or examples described herein may standon its own or may be combined in various permutations or combinationswith one or more of the other examples.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention may be practiced. These embodiments are also referred toherein as “examples.” Such examples may include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Method examples described herein may be machine or computer-implementedat least in part. Some examples may include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods may include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code may include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code may be tangiblystored on one or more volatile, non-transitory, or nonvolatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media may include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact discs and digital video discs), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments may be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments may be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The claimed invention is:
 1. A utility-scale energy storage andconversion system that adjusts a terminal voltage associated with afirst group of inverters and a second group of inverters, wherein thefirst group of inverters is associated with a first rating, wherein thesecond group of inverters is associated with a second rating that isgreater than the first rating, the utility-scale energy storage andconversion system comprising: a control circuit to: determine, using arepresentation of a first available reactive power value associated withthe first group of inverters and a representation of a second availablereactive power value associated with the second group of inverters, arepresentation of a total available reactive power; generate, using therepresentation of the first available reactive power value and arepresentation of a total required reactive power value, a first controlsignal representing a first desired reactive power value proportional tothe first available reactive power value; and generate, using therepresentation of the second available reactive power value and thetotal required reactive power value, a second control signalrepresenting a second desired reactive power value proportional to thesecond available reactive power value.
 2. The utility-scale energystorage and conversion system of claim 1, wherein the first group ofinverters associated with the first rating is associated with a firstplurality of batteries having the first rating, and wherein the secondgroup of inverters associated with the second rating is associated witha second plurality of batteries having the second rating.
 3. Theutility-scale energy storage and conversion system of claim 1, whereinthe first group of inverters includes a first number of inverters,wherein the second group of inverters includes a second number ofinverters different than the first number of inverters.
 4. Theutility-scale energy storage and conversion system of claim 1, thecontrol circuit to: determine, using a representation of a firstavailable apparent power value and a representation of a first requiredactive power value, the first available reactive power value, whereinthe first available apparent power value and the first required activepower value are associated with the first group of inverters; anddetermine, using a representation of a second available apparent powervalue and a representation of a second required active power value, thesecond available reactive power value, wherein the second availableapparent power value and the second required active power value areassociated with the second group of inverters.
 5. The utility-scaleenergy storage and conversion system of claim 1, wherein theutility-scale energy storage and conversion system forms part of anelectric network, and wherein the total required reactive power valuerepresents an amount of desired reactive power at a point ofinterconnect between the electric network and another electric network.6. The utility-scale energy storage and conversion system of claim 1,wherein the first group of inverters are coupled with a first voltagebus, wherein the second group of inverters are coupled with a secondvoltage bus, wherein the first voltage bus and the second voltage busare coupled with a third voltage bus through corresponding first step-uptransformers, wherein the third voltage bus is coupled with a fourthvoltage bus on an electric network through a second step-up transformer,wherein the fourth voltage bus of the electric network is coupled with apoint of interconnect coupled with another electric network, whereinvoltages of the first voltage bus and the second voltage bus are lessthan a voltage of the third voltage bus, and wherein the voltage of thethird voltage bus is less than a voltage of the fourth voltage bus.
 7. Amethod of adjusting a terminal voltage associated with a first group ofinverters and a second group of inverters, wherein the first group ofinverters is associated with a first rating, wherein the second group ofinverters is associated with a second rating that is greater than thefirst rating, the method comprising: determining, using a representationof a first available reactive power value associated with the firstgroup of inverters and a representation of a second available reactivepower value associated with the second group of inverters, arepresentation of a total available reactive power; generating, usingthe representation of the first available reactive power value and arepresentation of a total required reactive power value, a first controlsignal representing a first desired reactive power value proportional tothe first available reactive power value; and generating, using therepresentation of the second available reactive power value and thetotal required reactive power value, a second control signalrepresenting a second desired reactive power value proportional to thesecond available reactive power value.
 8. The method of claim 7, whereinthe first group of inverters associated with the first rating isassociated with a first plurality of batteries having the first rating,and wherein the second group of inverters associated with the secondrating is associated with a second plurality of batteries having thesecond rating.
 9. The method of claim 7, wherein the first group ofinverters includes a first number of inverters, wherein the second groupof inverters includes a second number of inverters different than thefirst number of inverters.
 10. The method of claim 7, furthercomprising: determining, using a representation of a first availableapparent power value and a representation of a first required activepower value, the first available reactive power value, wherein the firstavailable apparent power value and the first required active power valueare associated with the first group of inverters; and determining, usinga representation of a second available apparent power value and arepresentation of a second required active power value, the secondavailable reactive power value, wherein the second available apparentpower value and the second required active power value are associatedwith the second group of inverters.
 11. The method of claim 7, whereinthe total required reactive power value represents an amount of desiredreactive power at a point of interconnect between an electric networkand another electric network.
 12. The method of claim 7, wherein thefirst group of inverters are coupled with a first voltage bus, whereinthe second group of inverters are coupled with a second voltage bus,wherein the first voltage bus and the second voltage bus are coupledwith a third voltage bus through corresponding first step-uptransformers, wherein the third voltage bus is coupled with a fourthvoltage bus of an electric network through a second step-up transformer,wherein the fourth voltage bus of the electric network is coupled with apoint of interconnect coupled with another electric network, whereinvoltages of the first voltage bus and the second voltage bus are lessthan a voltage of the third voltage bus, and wherein the voltage of thethird voltage bus is less than a voltage of the fourth voltage bus. 13.A utility-scale energy storage and conversion system to use an on-loadtap changer (LTC) to adjust a terminal voltage associated with a firstgroup of inverters and a second group of inverters, wherein the firstgroup of inverters is associated with a first rating, wherein the secondgroup of inverters is associated with a second rating that is greaterthan the first rating, the utility-scale energy storage and conversionsystem comprising: a control circuit to: determine, using arepresentation of a measured current and a representation of a measuredvoltage at a transformer winding of a transformer, the terminal voltageassociated with the first group of inverters and the second group ofinverters; determine, using a representation of a total requiredreactive power value, a target terminal voltage associated with thefirst group of inverters and the second group of inverters; andgenerate, using a difference between the terminal voltage and the targetterminal voltage, a tap setting on the LTC so that the terminal voltageassociated with the first group of inverters and the second group ofinverters adjusts accordingly.
 14. The utility-scale energy storage andconversion system of claim 13, wherein the utility-scale energy storageand conversion system forms part of an electric network, wherein thefirst group of inverters are coupled with a first voltage bus, whereinthe second group of inverters are coupled with a second voltage bus,wherein the first voltage bus and the second voltage bus are coupledwith a third voltage bus through corresponding first step-uptransformers, wherein the third voltage bus is coupled with a fourthvoltage bus through a second step-up transformer, wherein the fourthvoltage bus of the electric network is coupled with a point ofinterconnect coupled with another electric network, wherein voltages ofthe first voltage bus and the second voltage bus are less than a voltageof the third voltage bus, wherein the voltage of the third voltage busis less than a voltage of the fourth voltage bus, wherein thetransformer is the second step-up transformer, and wherein therepresentation of the measured current and the representation of themeasured voltage are measured at a low-side of the second step-uptransformer.
 15. The utility-scale energy storage and conversion systemof claim 13, wherein the control circuit to determine, using therepresentation of the total required reactive power value, the targetterminal voltage associated with the first group of inverters and thesecond group of inverters is further configured to: determine the targetterminal voltage using a transfer function that relates the totalrequired reactive power value and the target terminal voltage.
 16. Theutility-scale energy storage and conversion system of claim 15, whereinat least a portion of the transfer function includes a linearrelationship between the total required reactive power value and thetarget terminal voltage.
 17. The utility-scale energy storage andconversion system of claim 15, wherein the control circuit configured todetermine the target terminal voltage using the transfer function thatrelates the total required reactive power value and the target terminalvoltage is configured to: determine the target terminal voltage using astored data set.
 18. The utility-scale energy storage and conversionsystem of claim 17, wherein the stored data set is a lookup table.